The acoustic sounder (SODAR) maps the amplitude of backscattered acoustic energy associated with temperature fluctuations and thus thermal inversions in the atmosphere. The aim of the SODAR measurements was to provide estimates of the height of the mixed layer and the vertical dimensions of inversions within the lower kilometer of the atmosphere. A single, vertically pointing, conventional SODAR was operated at an acoustic frequency near 1500 Hz to detect the amplitude of backscattered acoustic energy. The thickness of an elevated inversion as seen by the SODAR is often smaller than the difference between the heights of the inversion top and bottom, because of oscillations in the heights that occur. The heights were estimated only for the inversions that were clearly associated with the active mixed layer. These data were collected at one location in the northwest quadrant of the FIFE study area during the first three Intensive Field Campaigns held in 1987.